Starch-hydrolyzed polyacrylonitrile graft copolymers exhibiting the capacity to absorb from about 300 to 1,000 times their weight of deionized water are known at this time. The development of these compositions was carried out by the Northern Regional Research Laboratory, Peoria, Illinois. The starch-hydrolyzed polyacrylonitrile graft copolymer is produced by exposure of starch, either gelatinized or ungelatinized, to cerium salt, such as cerium ammonium nitrate, which acts as a catalyst to generate free radicals. Polyacrylonitrile chains form at the site of these free radicals. The resulting material is then saponified in sodium hydroxide to hydrolyze the polyacrylonitrile chains to carboxamide and alkali metal carboxylate groups mixed with metal salts. After drying, the material can absorb about 300 to 400 times its weight. Drying can be accomplished by drum, tumble air or vacuum drying. Extraction of the copolymer dispersion with alcohol before drying provides a material with fluid absorbency of 800 to 1,000 times its weight. The fluid absorbency is determined by suspending a weighed amount of dry copolymer in an excess of testing fluid and filtering to recover unabsorbed fluid.
The copolymer can be made as film, flakes, powder or mat. These forms take up water, swelling but not dissolving, and hold it in expanded duplications of their own dry shapes. Films extend and thicken to sheets. Powders become piles of water textured like crushed ice. A flake expands to a clear, angular piece of water. The swollen forms shrink in dilute acid and expand again in dilute alkali solution. They also shrink as they dry and expand again with water.
The copolymer, with these properties, can be mixed with or coated on a wide variety of materials including, for example, sand, straw, sawdust, seeds and roots, natural or synthetic fibers, and flour, gelatin and starch. It can hold water in soils, animal bedding and kitty litter, toweling and diapers, bandages, surgical pads and dental absorbents.
The copolymer, as commercially produced, is a granular powder. The powder varies in granulation, typically from 30 mesh to 400 mesh U.S. Standard Sieve. Fine powders such as those having granulation of 200 mesh to 325 mesh U.S. Standard Sieve, and especially those having a granulation of less than 325 mesh U.S. Standard Sieve, are not as generally useful as coarser granulations because they are harder to apply, harder to contain and, in some instances, they do not disperse or wet-out as well as the coarser particles. Finer granulations can be mechanically separated from the coarser. With respect to economics and material conservation, however, the finer material must be used in some manner.
A further disadvantage of the commercial product is the solvent content. Some alcohol residues are normally retained in the product upon drying. It is known that alcohol solvents used in purification of starch alone can be removed by passing gaseous water or humidified air through particles of the starch and, thereafter, drying the particles. Unfortunately, the fluid absorbency of the copolymer of this invention is adversely affected by these processes. The results of desolventizing with gaseous water or humidified air are shown in Examples I and II. In the past, therefore, there has been a problem in producing low alcohol content, high fluid absorbent copolymers suitable for uses in which alcohols could not be tolerated.